A variety of devices have been utilized in the prior art to translate the displacement of a point of a stylus into electrical signals indicative of the displacement of that stylus point along orthogonal axes on a plane or panel. Typical examples of such prior art devices are those that are described in U.S. Pat. No. 3,886,311 wherein a writing pen is utlized together with means for detecting time varying electrostatic field components created by the writing pen on a writing table capable of generating unique electrostatic fields. In such a device, means are provided for transmitting information related to and defined by the position of the sensor within the electrostatic field for determination of stylus point position. Other devices, such as those shown in U.S. Pat. No. 3,626,483 employ acoustic elements as the apparatus for position determination of a stylus tip. U.S. Pat. No. 3,911,215 shows a construction having an array of pressure sensors respective to pressure generated by a stylus tip for generation of voltage signals indicative of stylus position. A yet further type of device developed in the prior art is that disclosed in U.S. Pat. No. 3,790,709 wherein the instantaneous contact pressure of a probe on a piezoelectric plate provides a displacement-indicative signal. Additionally, electroacoustic sensors have been utilized which utilize a planar screen upon which an image or a pattern is projected, that when tapped at a particular point by a stylus, generates electrical signals representative of the coordinates in a plane of the point tapped. Such a device is disclosed in U.S. Pat. No. 3,857,022.
All of the above described devices utilized expensive panels which are structurally complex, which employ discrete sensor arrays and are limited in the determination of the exact position of a stylus point. Those which employ piezo electric, acoustic, or electrostatic sensing structures or which utilize resistance grids are subject to inaccuracies from temperature-induced drift, from changes in physical configuration due to temperature, as well as wear and deterioration where physical contact with the sensor, by the stylus, is necessary. In those devices where there is contact between stylus and surface, and those which employ optical sensors, cleanliness of the writing panel is also a problem.
With such drawbacks in mind, the present invention has as one of its objects creation of a simple structure lending itself to an inexpensive manufacturing process and increased resolution to enhance the fidelity of the device. A further object is the elimination of a sensitivity to wear encountered in many prior art devices. A still further object of the present invention is the reduction of possible environmental influences on performance, such as ambient temperature.
These as well as other objects and advantages are obtained by the graphic input device of the invention which utilizes a transparent flat panel, one side of which is rendered opaque, where a plurality of photoconductors are arrayed about the periphery thereof. This structure is simple, inexpensive to manufacture and capable of being utilized to provide analog output signals which in turn are capable of increasing overall resolution of the device. The individual photoconductors are arranged in a bridge circuit that also increases accuracy of the device and which is relatively stable to environmental influence. Furthermore, because the photoconductors age in a linear manner, an arrangement of them in a bridge configuration automatically compensates for aging and is also susceptible of adjustment to changes in operating characteristics. Because the sensors according to the invention may be arrayed about the periphery of a panel, and not imbedded therein, the physical construction of the graphic panel of the invention is simple and inexpensive to construct. Also, because the invention utilizes a point light source as a stylus or cursor, it is not susceptible to deterioration due to frictional wear nor is it susceptible to degradation resulting from lack of absolute panel cleanliness necessary in resistive type devices.